The present invention relates to a sputter target suitable for forming a barrier material for a semiconductor substrate or the like and to a barrier film and an electronic component using the same.
A storage device using a ferroelectric thin film as a storage medium, namely, a so-called ferroelectric memory (FRAM), has recently been under active development. The ferroelectric memory, which is nonvolatile, has such a characteristic that storage capacity thereof is not lost even after power source is cut off. Furthermore, spontaneous polarization inversion is very rapid if the film thickness of the ferroelectric thin film is sufficiently small, so that a rapid write and read comparable to DRAM can be realized. Since a memory cell of one bit can be constituted by a single transistor and a single ferroelectric capacitor, the ferroelectric memory is also suitable for mass storage.
As a ferroelectric material, lead zirconate titanate (a solid solution of PbZrO3 and PbTiO3 (PZT)) having a perovskite structure is mainly used. PZT, however, has such a disadvantage that its major component Pb is likely to be diffused and vaporized at a relatively low temperature (approximately 500xc2x0 C.), even though having such characteristics of a high Curie temperature (approximately 300xc2x0 C.) and large spontaneous polarization, and therefore, it is said to be difficult for PZT to cope with miniaturization. Barium titanate (BaTiO3 (BTO)) is known as a typical ferroelectric material besides PZT. BTO, however has such a disadvantage that remanent polarization thereof is greatly temperature-dependent due to small remanent polarization and a low Curie temperature (approximately 120xc2x0 C.) compared with PZT.
It has been found out, however, that BTO, when epitaxially grown on a Pt/MgO(100) substrate, allows a BTO film having a film thickness of, for example, 60 nm to exhibit the Curie temperature of 200xc2x0 C. or higher. Moreover, it has been confirmed that, when barium strontium titanate (BaaSr1-aTiO3 (BSTO)) is epitaxially grown on a lower electrode made of Pt and strontium ruthenate (SrRuO3 (SRO)), ferroelectricity appears in a composition region (axe2x89xa60.7) which is not expected to exhibit the ferroelectricity by nature. This is because a lattice of a BSTO crystal in a C-axis direction is extended.
Since a ferroelectric Curie temperature shifts to a higher temperature side in such a BSTO film of a Ba rich, large remanent polarization is obtainable in a room temperature zone, and sufficiently large remanent polarization can be retained even when the temperature is increased up to approximately 85xc2x0 C. Consequently, a ferroelectric film suitable for the storage medium of FRAM can be realized. Meanwhile, the use of BSTO of an Sr rich can realize a thin-film capacitor whose dielectric constant reaches several times (for example, 800 or higher) as that of a capacitor made of a polycrystalline film. Such a dielectric property is suitable for DRAM.
The practical availability of semiconductor memories such as FRAM and DRAM is expected through the use of the thin-film capacitor having the epitaxially grown BTO film and BSTO film as described above. In putting these into practical use, it is necessary to combine a semiconductor substrate on which a switching transistor is formed and a memory cell using a perovskite oxide film (thin-film capacitor). At this time, a problem exists that the diffusion of elements such as Pt, Ru, Sr, and Ba, which constitute the lower electrode and the dielectric thin film of the thin-film capacitor, into the transistor has an adverse effect on a switching operation.
Under the circumstances, a barrier film which prevents mutual diffusion needs to be formed between the thin-film capacitor and the semiconductor substrate. Further, the barrier film itself needs to be epitaxially grown on the semiconductor substrate in order to obtain the above-described epitaxial effect. The use of a titaium nitride (TiN) film and a film made of Ti1-xAlxN(Tixe2x80x94Alxe2x80x94N) which is a solid solution of TiN and aluminum nitride (AlN) has been studied as such a barrier film.
TiN, which is superior in a barrier property against Al and the like, is also utilized as a barrier metal in generally-used Si devices. It is also excellent in thermal stability since it is a chemical compound whose melting point is high (3000xc2x0 C. or higher), and has a very low specific resistance, approximately 50 xcexcxcexa9xc2x7cm in a polycrystalline film and approximately 18 xcexcxcexa9xc2x7cm in an epitaxial film, which results in an advantage that contact resistance can be lowered in utilizing an electric property in the thickness-wise direction.
When TiN is used as the barrier film of the thin-film capacitor, however, oxygen is diffused onto the TiN film, for example, due to annealing at a high temperature (for example, 600xc2x0 C. or higher) conducted in an element production process for controlling crystallization of the ferroelectric film so that nitrogen (N) in TiN is substituted by oxygen (O) to form an oxide film, namely, TiO2. The lower electrode made of Pt, SRO, and so on becomes inferior in adherence due to volume expansion thereof based on TiO2 generated on the surface of the TiN film and due to the generation of N2 gas. This results in a problem that peeling occurs in the lower electrode.
When Al is added to TiN to form the Ti1-xAlxN(Tixe2x80x94Alxe2x80x94N) film, oxidation resistance can be enhanced. The Tixe2x80x94Alxe2x80x94N film is formed by reactive sputtering in an atmosphere of argon (Ar) and nitride (N), using a Ti1-xAlx alloy (a Tixe2x80x94Al alloy) target. Concerning the Tixe2x80x94Al alloy target, for example, Japanese Patent Laid-open Application No. Hei 6-322530 specifies a Tixe2x80x94Al alloy target constituted only of a diffusion reaction layer of high-purity Ti and high-purity Al.
Further, aiming at enhancing abrasion resistance and oxidation resistance of cutting tools, sliding components, and so on, Japanese Patent Laid-open Application No. Hei 8-134635 specifies a Tixe2x80x94Al alloy target material with a relative density of 99.0 to 100% and free of any continuous defect from the surface to the bottom surface thereof. Japanese Patent Laid-open Application No. 2000-100755 specifies a Tixe2x80x94Al alloy target for forming a barrier film of a semiconductor device, whose O content is in the range of 15 to 900 ppm.
Further, Japanese Patent Laid-open Application No. 2000-273623 specifies a Tixe2x80x94Al alloy target, in which an Al content is 5 to 65 wt %, a radio active element such as U and Th is 0.001 ppm or lower, an alkali metal such as Na and K is 0.1 ppm or lower, Fe which is a transition metal is 10.0 ppm or lower, Ni is 5.0 ppm or lower, Co is 2.0 ppm or lower, Cr is 2.0 ppm or lower, and purity thereof including impurities is 99.995% or higher, and Japanese Patent Laid-open Application No. 2000-328242 specifies a Tixe2x80x94Al alloy target containing 15 to 40 atm % or 55 to 70 atm % of Al and having a metal structure with an area ratio of a Ti3Al intermetallic compound being 30% or higher, and in which the number of defects with a diameter of 0.1 mm or larger is 10/100 cm2 or less. Thus, various kinds of Tixe2x80x94Al alloy targets have been developed.
The Tixe2x80x94Alxe2x80x94N film which is formed by reactive-sputtering the conventional Tixe2x80x94Al alloy target, however, is inferior in an epitaxial growth property on an Si substrate, which results in a problem of hindering the epitaxial growth of the BTO film and the BSTO film. In FRAM using such a BTO film or a BSTO film, a ferroelectric property such as remanent polarization is not sufficiently obtainable to lower the property and production yields of FRAM. When they are applied to DRAM, the property and production yields thereof are similarly lowered as well.
Further, when the Tixe2x80x94Alxe2x80x94N film is formed by reactive-sputtering the conventional Tixe2x80x94Al alloy target, sudden generation of huge dust is likely to occur while the film is formed by sputtering, which results in a problem of lowering the production yields of FRAM and DRAM. Such a problem is caused not only when the Tixe2x80x94Alxe2x80x94N film is used as the barrier film of the thin-film capacitor but also when the Tixe2x80x94Alxe2x80x94N film is used as the barrier film of a generally-used semiconductor element.
As described above, though the Tixe2x80x94Alxe2x80x94N-film has a characteristic of being excellent in oxidation resistance by nature, it cannot be necessarily said that sufficient studies have been made on the composition, nature, and so on of the Ti1-xAlx alloy target used for the formation thereof. This is why such problems occur that the epitaxial growth property of the Tixe2x80x94Alxe2x80x94N film on the Si substrate is degraded and in addition, the sudden generation of the huge dust is caused.
An object of the present invention is to provide a sputter target enabling the formation of a Tixe2x80x94Alxe2x80x94N film excellent in the property and quality as a barrier film with good reproducibility. More specifically, an object of the present invention is to provide a sputter target enabling the epitaxial growth of the Tixe2x80x94Alxe2x80x94N film with good reproducibility, and a sputter target enabling the reduction in the dust generation. Another object is to provide, through the use of such a sputter target, a barrier film and an electronic component whose property, quality and production yields are enhanced.
As a result of studies, with the aim of solving the above-described objects, on the influence that Al composition, a crystal grain diameter, and so on in a Tixe2x80x94Al alloy target give to a Tixe2x80x94Alxe2x80x94N film, the inventors of the present invention have found out that it is possible to enhance an epitaxial growth property of the Tixe2x80x94Alxe2x80x94N film and to reduce dust generation when Al in an Tixe2x80x94Al alloy is first solid-solubilized in Ti or is made to exist as an intermetallic compound with Ti to obtain a uniform alloy structure (target structure).
It has been found out that especially the epitaxial growth property of the Tixe2x80x94Alxe2x80x94N film is greatly enhanced by reducing variation in Al content in the entire target. In other words, reduction in segregation of Al enhances the epitaxial growth property of the Tixe2x80x94Alxe2x80x94N film. Meanwhile, it has been found out that the dust generation is greatly reduced by reducing variation in crystal grain diameter in the entire target.
The present invention is made based on the above findings. A first sputter target of the present invention is a sputter target comprising a Tixe2x80x94Al alloy, characterized in that Al in the Tixe2x80x94Al alloy exists in at least one of a solid solution state in Ti and a state in which Al forms an intermetallic compound with Ti, and that variation in Al content in the entire target is within 10%.
A second sputter target of the present invention is a sputter target comprising a Tixe2x80x94Al alloy, characterized in that Al in the Tixe2x80x94Al alloy exists in at least one of a solid solution state in Ti and a state in which Al forms an intermetallic compound with Ti, that an average crystal grain diameter of the Tixe2x80x94Al alloy is 500 xcexcm or smaller, and that variation in the crystal grain diameter in the entire target is within 30%.
In the sputter target of the present invention, the Tixe2x80x94Al alloy preferably contains Al in the range of 1 to 30 atm %.
A barrier film of the present invention is characterized in that it comprises a Tixe2x80x94Alxe2x80x94N film formed by using the sputter target of the present invention described above. The barrier film of the present invention is suitably used as a barrier material for a semiconductor substrate.
An electronic component of the present invention is characterized in that it comprises the barrier film of the present invention described above. As a concrete form of the electronic component of the present invention, a semiconductor memory comprising a semiconductor substrate, the barrier film formed on the semiconductor substrate, and a thin-film capacitor formed on the barrier film can be named.